1. Field of the Invention
The present invention relates to an objective lens system, and more particularly to an immersion planapochromat-grade medium-power microscope objective.
2. Related Background Art
Conventional immersion microscope objectives are capable of being used for only a type of liquid. Consequently, if observing objects each are adaptive to different liquids, for example one to water and another to oil, the objective must be changed one from another so as to match each of the observing objects.
Thus, there are demands for an immersion microscope objective that can be used for some types of liquids having different refractive indices, for example water, oil, and glycerin. However, because water, oil, and glycerin have respective indices ranging from about 1.3 to about 1.6, aberrations remain as not being corrected well.
An example of such conventional immersion apochromat-grade objectives is the objective as disclosed in the bulletin of Japanese Laid-open Patent Application No. 59-155822. The objective disclosed in this bulletin uses a cemented lens including an embedded lens as a front lens (a lens closest to the object). By properly setting a radius of curvature of an interface (cemented surfaces) of the cemented lens and an index difference between two component lenses in the cemented lens, the Petzval sum is decreased to correct the curvature of field.